In-line light sensor

ABSTRACT

An integrated optical waveguide ( 1 ) having an in-line light sensor ( 2 ) integrally formed therewith for tapping off a small proportion of the signal transmitted along the waveguide ( 1 ). A first part ( 1 A) of the waveguide leads to a photodiode portion ( 2 ) and a second part ( 1 B) of the waveguide leads away from the photodiode portion ( 2 ), the photodiode portion ( 2 ) comprising one or more regions ( 14 ) of light absorbing material within the waveguide ( 1 ) arranged to absorb a minor proportion of light transmitted along the waveguide ( 1 ) and thereby to generate free charge carriers within the waveguide ( 1 ). Deep band gap levels ( 21 ) are introduced into photodiode portion by ion implantation to enable it to absorb selected wavelengths. The free charge carriers are detected by a p-i-n diode formed across the waveguide ( 1 ). Wavelength selective reflectors ( 11, 12 ) may be provided either side of the photodiode portion ( 2 ) so light passes repeatedly through the photodiode portion ( 2 ).

TECHNICAL FIELD

This invention relates to an in-line light sensor, in particular a lightsensor for sensing an optical signal transmitted along an integratedoptical waveguide.

BACKGROUND ART

A variety of types of light sensors are known which can be mounted on anintegrated optical circuit in order to receive light from a waveguideintegrated on the circuit. One example is a SiGe/Si multi-quantum well(MQW) structure arranged to form a photodetector which can be mounted ona silicon optical circuit to receive an optical signal directed theretoby a waveguide.

The present invention aims to provide a light sensor having advantagesover such known light sensors.

SUMMARY OF INVENTION

According to a first aspect of the invention, there is provided anintegrated optical waveguide having an in-line light sensor integrallyformed therewith, comprising: a first part of the waveguide leading to aphotodiode portion thereof; a second part of the waveguide leading awayfrom the photodiode portion, the photodiode portion comprising one ormore regions of light absorbing material within the waveguide arrangedto absorb a minor proportion of light of one or more selectedwavelengths transmitted along the waveguide and thereby to generate freecharge carriers within the waveguide; and detecting means for detectingthe presence of said free charge carriers.

According to a second aspect of the invention, there is provided amethod of fabricating such a waveguide in which the photodiode portionand the detecting means are fabricated by one or more of the following:photolithographic techniques, doping and ion implantation.

According to another aspect of the invention, there is provided anintegrated optical waveguide having an in-line light sensor integrallyformed therewith the light sensor comprising a p-i-n diode formed in asemiconductor substrate having an energy band gap the magnitude of whichcorresponds to absorption of photons of a first wavelength, thephotodiode comprising a substantially intrinsic region in saidsemiconductor substrate between p- and n-doped regions, the intrinsicregion being modified to introduce deep band gap levels therein so as toprovided at least partial absorption of photons of an optical signal ofa selected wavelength or wavelength band greater than said firstwavelength and thus generate an electrical signal across the p-i-n diodeindicative of said optical signal, said photodiode being provided withina resonant cavity.

Preferred and optional features of the invention will be apparent fromthe following description and from the subsidiary claims of thespecification.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be further described, merely by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic plan view of a waveguide with an in-line lightsensor according to a preferred version of the invention;

FIG. 2 is a schematic cross-sectional view through a first embodiment ofsuch an in-line light sensor;

FIG. 3 is a cross-sectional view through a second embodiment of such anin-line light sensor;

FIG. 4 is a schematic, cross sectional view of an in-line light sensoraccording to a third embodiment of the invention;

FIG. 5 is a band-gap diagram illustrating the operation of a lightsensor such as that shown in FIG. 4;

FIGS. 6A to 6F illustrates steps in a preferred method of fabricating alight sensor such as that shown in FIG. 4;

FIG. 7 shows a modified form of the device shown in FIG. 4;

FIG. 8 is a cross-sectional view through a fourth embodiment of anin-line light sensor;

FIG. 9 is a cross-sectional view through a fifth embodiment of anin-line light sensor;

FIG. 10 is a plan view of another embodiment of an in-line light sensoraccording to the invention;

FIG. 11 is a perspective view of a further embodiment of an in-linelight sensor according to the invention;

FIG. 12A is a perspective view of yet a further embodiment of an in-linelight sensor according to the invention and FIG. 12B is across-sectional view thereof;

FIG. 13 is a perspective view of a light sensor according to anotheraspect of the invention;

FIG. 14 is a schematic plan view of an in-line light sensor such as thatshown in FIG. 2;

FIG. 15 is a cross-sectional view taken along line A-A′ of FIG. 14;

FIG. 16 shows this type of light sensor modified according to apreferred embodiment of the invention;

FIG. 17 is a perspective view of a Bragg grating as may be used in thearrangement shown in FIG. 16;

FIG. 18 is a schematic plan view of a known form of optical channelmonitor;

FIG. 19 is a schematic plan view of a series of light sensors accordingto the present invention arranged to monitor a plurality of wavelengths;

FIG. 20 is a schematic diagram illustrating a typical known arrangementof an optical channel monitor;

FIG. 21 is a schematic plan view of a serpentine version of the seriesshown in FIG. 19; and

FIG. 22 is a further embodiment of the modified light sensor shown inFIG. 16.

BEST MODE OF THE INVENTION

FIG. 1 shows a waveguide 1 which forms part of an integrated opticalcircuit (not shown) formed on a planar substrate, e.g. a silicon chip. Afirst part 1A of the waveguide receives an input signal and transmitsthis to a portion of the waveguide which is arranged to provide anin-line light sensor in the form of a photodiode 2 and a second part 1Bof the waveguide receives the signal leaving the photodiode 2 anddirects this to an output.

The in-line photodiode 2 is arranged to be partially transparent so thatit absorbs only a minor proportion of the light transmitted along thewaveguide 1 in a single pass therethrough. The photodiode 2 comprisesone or more light absorptive regions which absorb part of the signalbeing transmitted along the waveguide. In a typical arrangement, theabsorptive regions may, for instance, absorb 5% or less of the signalbeing transmitted along the waveguide (in a single pass therethrough).

The degree of absorption of the signal may be controlled by thedimensions of the absorptive region, e.g. by its length along thewaveguide and depth (perpendicular to the plane of the chip), and/or bycontrolling the absorption coefficient thereof.

In a typical arrangement, the absorptive region may, for example, have alength in the range 100-10,000 microns and a depth of around 0.1microns. If the optical interaction between the light signal and theabsorptive region is relatively small, a relatively long interactionlength (i.e. a long photodiode region) can be used to ensure aphotocurrent of the desired magnitude is produced.

The absorptive region is arranged to generate free charge carriers whenlight of one or more selected wavelengths is incident thereon and theseare detected by a diode formed across or within the waveguide as will bedescribed further below.

The photodiode absorptive region(s) may comprise any material whichgenerates free charge carriers when light of one or more selectedwavelengths is incident thereon. The photodiode region may, forinstance, comprise a semiconductor material having a band gap of a sizesuch that photons of a given wavelength (or shorter wavelengths) areable to excite charge carriers across the band gap from the valence bandto the conduction band. Alternatively, it may comprise a semiconductormaterial whose band gap is too large for this to occur for thewavelength(s) of interest but in which deep band gap levels are formedbetween the conduction and valence bands to facilitate the generation offree charge carriers upon illumination by such wavelengths. It may alsocomprise light absorptive material such as polycrystalline or amorphoussemiconductor materials.

The term “deep band gap levels” as used herein refers to states betweenthe valence band and conduction band of the semiconductor material butspaced therefrom by a sufficient energy gap such that thermal excitationof electrons from the valence band to the deep band gap state or fromthe deep band gap state to the conductive band is small. It will beappreciated that the magnitude of this energy gap will depend upon thetemperature of the device, and, whilst the present invention is notlimited to temperature, it is primarily directed towards device designedto operate in the temperature range of 0 to 100 degrees Centigrade.

FIGS. 2 and 3 shows cross-sections through in-line photodiodes formedwithin rib waveguides 10 fabricated in an optically conductive siliconlayer 11. Preferably, the silicon layer 11 is separated from asupporting substrate 12 (typically also of silicon), by an opticalconfinement layer 13 (typically of silicon dioxide). Such a structure isconveniently formed from a silicon-on-insulator chip (as widely used forintegrated electrical circuits).

FIG. 2 shows an absorptive region 14 formed within a rib 10A of the ribwaveguide 10, the rib projecting from a slab region 10B of the siliconlayer 11.

In this example, the absorptive region 14 is spaced from the uppersurface of the rib 10A leaving a silicon buffer region 10C therebetweento provide the desired overlap between the absorptive region 14 and theoptical mode transmitted along the waveguide (the approximate locationof the mode profile of a signal guided by the rib waveguide is shown bydashed lines 16). An oxide layer 15 is provided over the silicon layer11 for optical confinement and electrical insulation.

When an optical signal is transmitted along the waveguide, part of thesignal will be absorbed by the absorptive region 14. This generates freecharge carriers within the waveguide and these are detected by a lateralp-i-n diode formed across the waveguide. The p-i-n diode comprises ap-doped region 17 and an n-doped region 18 formed in the silicon layer11 on opposite sides of the rib waveguide with a nominally intrinsicregion therebetween. Metal contacts 19 and 20 are provided to provideelectrical connection to the p- and n-doped regions 17 and 18,respectively.

P-i-n diodes are known for injecting charge carriers into a waveguide toalter its refractive index and to attenuate optical signals therein butin this case the p-i-n diode is used to collect charge carriersgenerated by the partial absorption of an optical signal by absorptiveregion 14 and thus provide an electrical signal indicative of theoptical signal within the waveguide.

In the embodiment shown, the p and n-doped regions 17 and 18 are formedbeneath recesses 21 and 22 formed in the silicon layer 11. This helpsensure the p- and n-doped regions extend through the silicon layer 11 tothe oxide layer 13 and allows a smaller structure to be formed withoutincreasing optical losses caused by the presence of the doped regionsadjacent the waveguide. The collection efficiency of the photogeneratedcharge carriers is also improved (as is the speed of the device) byreducing the distance between the absorptive region 14 and the dopedregions 17 and 18.

The dimensions of a waveguide are typically in the range 1-10 microns sothe distance the charge carriers have to move to be collected by the p-or n-doped regions 17 and 18 from the locations where they are generated(in the absorptive region 14) is typically 10 microns or less in sucharrangement and preferably 5 microns or less. This not only increasesthe speed of the device, i.e. the time between the optical signal beingreceived by the absorptive region and the generation of an electricalsignal across the p-i-n diode is very short, but also reduces theopportunity for impurities or other materials in the silicon layer fromabsorbing the charge carriers (and thus preventing them from reachingthe p- or n-doped regions 17 and 18).

As the light sensor is intended to monitor the signal being transmittedalong the waveguide, the absorptive region 14 is preferably located in aposition where it only partially overlaps the optical mode 16. In theillustrated example, the absorptive region 14 is located within the rib10A towards the uppermost portion thereof. Alternatively, oradditionally, one or more absorptive regions could be provided in theslab region 10B on one or both sides of the rib waveguide, i.e. betweenthe rib 10A and the doped region 17, 18, or at the bottom of the siliconlayer 11 adjacent the oxide layer 13.

In other arrangements, depending upon the nature of the absorptiveregion and the degree of absorption required, the absorptive region maybe positioned so as to maximise the overlap with the optical mode, e.g.as shown in FIG. 4 (described further below).

FIG. 3 shows an alternative arrangement which is similar to that of FIG.2 except that the p-i-n diode is formed between p-doped regions 30, 31on one or both sides of the rib waveguide 10 and an n-doped region 32provided in the upper surface of the rib 10A (or vice versa). Such avertical arrangement may be more appropriate with thicker waveguides(measured perpendicular to the plane of the chip).

In many applications, it is desirable to sense optical signals of aselected wavelength or wavelength band. Discrete silicon based p-i-nphotodiodes are used in the opto-electronics industry. However, theirefficiency is greatest in the wavelength region 0.6-0.8 microns (whichcorresponds to energies above the band-gap of silicon).

Silicon based optical circuits are now being produced and these requirephotodiodes for sensing optical signals within the circuits. To date,such photodiodes have been formed of other materials, e.g. InGaAs/Ge,which are capable of sensing the wavelengths of 1.3 and 1.5 micronscommonly used in opto-communication devices and/or are of such aconstruction that the diode has to be hybridised with the siliconcircuit, i.e. mounted thereon as a separate component.

FIG. 4 shows a schematic, cross-sectional view of a p-i-n photodiodeformed in a silicon substrate somewhat similar to that of FIG. 2 wherethe silicon substrate comprises a silicon-on-insulator chip having asilicon light conducting layer 21 separated from a substrate 22, whichis also typically of silicon, by a light confining layer 23, e.g. ofsilicon dioxide.

A rib 24 is formed in the silicon layer 21 so as to form a rib waveguidetherein. The approximate position of the optical mode within thiswaveguide is illustrated by the dashed line 25.

Recesses 21A and 21B are preferably formed in the silicon layer 21 andn- and p-doped regions 26 and 27 formed in the silicon layer 21 bydoping through these recesses. Electrical contacts 28 and 29 connect then- and p-doped regions to an electrical circuit 20 arranged to provide areverse bias between the n- and p-doped regions. In some cases, areverse bias may not be applied and the electrical circuit just used todetect the photocurrent generated.

Other doping arrangements for the p-i-n diode are possible, e.g. withoutthe recesses 21A and 21B or side-doped arrangements as described in WO00/10039.

The layer 21 of silicon between the n- and p-doped regions issubstantially intrinsic so this arrangement forms a p-i-n diode acrossthe rib waveguide. However, the intrinsic region is modified tointroduce deep band gap levels therein as indicated by the Xs in region21C of FIG. 4. These levels can be excited by sub-band-gap photons soproducing free charge carriers and thus a measurable electrical signalin the circuit 20 upon application of a voltage across the p-i-n diode.

Deep band gap levels are preferably introduced by ion implantation whichcan give rise to free charge carriers in several ways:

-   (i) ion implantation induced defects (in the crystalline structure)    in the intrinsic material. In this case, charge carriers may be    excited into the deep-gap states upon illumination by light of the    appropriate wavelength and then undergo “hopping” conduction from    defect to defect.-   (ii) ion implantation induced defects in doped material, the defects    resulting from the implantation of non-dopant atoms such as silicon,    helium or hydrogen. Charge carriers will be trapped in the deep-gap    defect states and released to the conduction band upon illumination    by light of the appropriate wavelength.-   (iii) ion implantation and subsequent activation by heat treatment    of deep-level impurities, e.g. gold, oxygen or carbon atoms. The    heat treatment typically involves heating to 650-1000 degrees C. for    several seconds. The impurity atoms occupy sites in the crystal    lattice and the electrons associated therewith form the deep band    gap states. The heat treatment re-crystallises the silicon to repair    the damage caused by the implantation of the impurities. These    impurities thus form substitutional dopants which distort the    crystal lattice.

The formation of deep band gap levels and the mechanisms by which theygive rise to free charge carriers are known so will not be described ingreater detail.

Impurities which are used as substitutional dopants should preferablyhave the following properties:

-   -   a) they should give rise to deep levels within the band gap at        the required energy level(s).    -   b) have relatively low diffusion rates (to provide control over        their location and provide stability, i.e. lack of drift in        properties during use).    -   c) be reasonably soluble so a sufficient concentration can be        implanted.    -   d) have a sufficiently strong optical interaction so as to        provide a detectable photocurrent.

The use of ion implantation to form the deep band gap levels allowsprecise control of their depth and concentration so the amount of lightabsorbed can be tightly controlled. In the example shown in FIG. 2, thedeep band gap levels are formed at a depth which corresponds with thelocation of the optical mode 5 so as to maximise the interactiontherewith. However, as mentioned above, in other arrangements the deepband gap levels may be located so as to interact with only part of theoptical mode.

FIG. 5 is a band-gap diagram illustrating operation of the device. Thelines 30 and 31 represent the energy levels of the band gap from thep-doped region to the n-doped region when the p-i-n diode is underreverse bias. The Xs again represent the deep band gap levels and arelocated in the intrinsic region between the band-gap energy levels.Charge carriers absorbing incoming photons (indicated by hν) can beexcited from the lower energy level 30 to the deep-sub-band-gap levelsas indicated by vertical arrow 32A and then take part in hoppingconduction. Alternatively, carriers absorbing incoming photons can beexcited from the deep-sub-band-gap levels to the upper energy level 31as indicated by vertical arrow 32B.

A p-i-n photodiode such as that described above can be integrated withan optical circuit formed on the silicon-on-insulator chip usingconventional fabrication techniques, e.g. photolithography, dopingand/or ion implantation, used in the fabrication of other features ofthe integrated optical device. The need to hybridise a photodiode ontothe silicon chip is thus avoided.

FIGS. 6A-6F illustrate a method by which a photodiode such as thatdescribed above may be fabricated.

FIG. 6A illustrates a rib 44 etched in the silicon layer 41 of asilicon-on-insulator chip.

FIG. 6B shows the formation of p-and n-doped regions 46 and 47 byconventional doping techniques.

A mask 40, e.g. of silicon dioxide is then formed over the device and awindow 41 formed in the mask to expose the upper surface of rib 44 asshown in FIG. 6C.

FIG. 6D illustrates the ion implantation step through the window 41 inthe mask 40. Ions such as Si⁺, He⁺ or H⁺ may be implanted to createdamage in the crystalline silicon layer 41, e.g. vacancy pairs andhigher order vacancy clusters. Such damage in turn createsdeep-sub-band-gap states allowing subsequent detection ofdeep-sub-band-gap light as described by mechanisms (i) and (ii) above.H⁺ implantation, i.e. proton implantation, is preferred in some casesbecause it allows the defects to be formed at any desired depth withinthe waveguide. Alternatively, ions such as Au⁺, O⁺ or C⁺ may beimplanted. These ions, once electrically activated, createdeep-sub-band-gap states allowing subsequent detection ofdeep-sub-band-gap light as described in mechanism (iii) above.

The mask 40 is then removed and the device thermally treated to eitherengineer the defect structure (in the case of, e.g., Si⁺, He⁺ or H⁺implantation) or electrically activate the deep electrical dopants (inthe case of, e.g., Au⁺, C⁺ or O⁺ implantation). In the case of defectengineering, this typically involves heat treatment at relatively lowtemperatures, e.g. between 25 and 800 degrees C. for several seconds. Inthe case of electrical activation (i.e. activation of the p- orn-dopant), this will typically involve heat treatment at temperaturesbetween 650 and 1000 degrees C. for several minutes. The heat treatmentalso stabilises the device by removing defects which may otherwiseslowly dissipate over time (leading to drift in device performance). Thedeep-sub-band gap defects thus produced are indicated by Xs in FIG. 6E.

Finally, electrical contacts 48 and 49 are formed and the deviceconnected to an electrical circuit 40 as shown in FIG. 6F.

As discussed above, when an optical signal of a selected wavelength orwavelength band, in this case the 1.3 and 1.5 micron bands commonly usedin opto-communication applications, is transmitted along the ribwaveguide an electrical signal is generated by charge carriers movingeither to or from the deep-sub-band-gap levels either to or from theconduction valance bands as illustrated in FIG. 5 which can be detectedby the electrical circuit 10 to provide an output indicative of theoptical signal.

A photodiode capable of detecting photons of a wavelength greater thanthat corresponding to the band gap of silicon can thus be provided byforming defects in the intrinsic region of a p-i-n photodiode andengineering these defects to provide deep-sub-band-gap levels in theintrinsic region.

Other geometric arrangements of the p-i-n photodiode structure may alsobe used. FIGS. 2 and 4 illustrate lateral p-i-n photodiode formed acrossa rib waveguide. A vertical p-i-n structure may also be used, e.g. byforming p-doped regions on each side of the waveguide and an n-dopedregion on the top of the rib of the waveguide, or vice versa, as shownin FIG. 3. In other cases, a longitudinal p-i-n diode may also be used.

FIG. 7 shows a modified form of the p-i-n photodiode shown in FIG. 4. Asdescribed above, the silicon layer 21 is nominally intrinsic. Inpractice, this means its level of dopant which is low enough to avoidsignificant optical loss. Typically such “intrinsic” material has adopant level of around 10¹⁴ to 10¹⁵ cm⁻³, optical losses usually onlybeing of concern at levels of 10¹⁶ cm⁻³ or higher. In contrast, thedoped regions 26, 27 typically have a dopant level of around 10¹⁹ cm⁻³.

The presence of dopant in the intrinsic material means that a pn diodeis in effect formed at the interface between one of the doped regions26, 27 and the intrinsic region. If, for instance, the intrinsic regionis, in effect, lightly p-doped, a pn junction will be formed at theinterface between the n-doped region 26 and the intrinsic region betweenthe doped regions 26 and 27. Thus, when free charge carriers are createdby photoabsorption in the region 21C of the deep band gap levels, theyhave to reach the pn junction at the interface between the n-dopedregion 26 and the intrinsic region before being detected. If thisdistance can be reduced, the response time will be improved and thelikelihood of the charge carrier being absorbed before it is detectedwill also be reduced, i.e. the photoresponse will be increased.

In FIG. 7, the intrinsic region is modified by forming it so that onehalf 21D adjacent the n-doped region 26 is lightly n-doped and the otherhalf 21E adjacent the p-doped region 27 is lightly p-doped. This givesrise to a pn diode in the centre of the device (indicated by dotted line21F in FIG. 7) between the two lightly doped halves of the intrinsicregion. This pn junction is located within the region 21C where the freecharge carriers are generated so helps improve the response time of thedevice and reduces the likelihood that the charge carriers are absorbedbefore being detected.

If the intrinsic region is lightly p-doped to start with, the abovearrangement can be produced by lightly doping one half of the devicewith n-dopant. Other ways of producing such an arrangement will beapparent to those skilled in the art. As the intrinsic region is stilllightly doped, problems associated with optical loss due to absorptionof the optical signal by dopant are avoided, especially if the dopantlevel is kept below 10¹⁶ cm⁻³.

The dimensions of the rib waveguide are typically in the range of 1-20microns. The charge carriers generated within the photodiode thereforeonly need to be swept over relatively short distance, e.g. 10 microns orless or preferably 5 microns or less, before being detected by the p- orn-doped regions.

The deep-sub-band gap levels may also be formed at any position orpositions within the intrinsic region. For instance, instead of beingcentrally located as shown in FIG. 6F they may be located either side ofthe region, e.g. in the slab regions between the rib 44 and the dopedregions 46 and 47, as indicated above.

The nature of the arrangements described above also allows thephotodiode to be fabricated easily and extremely accurately and for itsproperties to be carefully tailored to suit the application. The energylevels of the deep band gap levels can be accurately determined as wellas their location within the device. The formation of the deep band gaplevels by ion implantation allows precise control of the depth andconcentration of the deep band gap states so the wavelength and amountof light absorbed can be tightly controlled.

Also, whilst the invention has been described above in relation to thep-i-n photo-diode formed in silicon, the invention can also be appliedto other semiconductor material in which deep band gap levels can beintroduced to enable wavelengths longer than that associated with theband gap of the material to be detected. The examples given above relateto light sensors integrated on a silicon substrate but the principle isapplicable to other types of substrate, e.g. in III-V compounds such asInGaAsP or InP.

When silicon is used as the semiconductor substrate the arrangementsdescribed enable the photodiode to be integrated within a silicon-basedintegrated optical circuit. The need to hybridise photodiodes made ofother semiconductor materials onto a silicon substrate is thus avoided.

As indicated above, deep-sub-band-gap levels may be formed in variousways. One method is to form defects in the crystalline structure ofsilicon itself, e.g. caused by the implantation or bombardment of thesilicon by hydrogen, helium or silicon atoms. Or, as mentioned above,another method is the introduction of electrical impurities such asgold, oxygen or carbon atoms.

Other absorptive materials may also be used. Examples of suitableabsorptive materials include: Si/Ge alloys, Ge-rich regions within asilicon matrix, polycrystalline silicon, amorphous silicon, ironsilicide, etc.

In some cases, the absorptive region may be arranged to absorb aspecific wavelength or wavelength band, e.g. wavelengths of around 1.3and/or 1.5 microns (as in the examples given above), in other cases theabsorptive region may be capable of absorbing a wider range ofwavelengths.

The absorptive region(s) is selected so as to be suitable for beingintegrally formed with or in the waveguide. This greatly facilitates themanufacture of the light sensor as it is then not necessary to hybridisethe light sensor as a separate component onto the optical circuit.

In another alternative, the absorptive region may comprise a SiGe layer,e.g. formed by selective epitaxial growth on the rib of the ribwaveguide. Similarly, other types of absorptive regions may be used,e.g. Ge-rich islands, Si-amorphous regions, and FeSi₂.

If amorphous silicon is used as the absorptive region, this may beprovided at the upper portion of the rib of the rib waveguide without anintervening layer of silicon. A layer of amorphous silicon can befabricated in the rib by ion implantation.

Amorphous silicon absorbs light at a wavelength of 1.55 microns and thedimensions of the amorphous layer, i.e. its thickness and/or length, canbe adjusted to provide the required degree of absorption.

A further advantage of the in-line photodiodes described above is thatthey have low polarisation dependency. Even if the materials used havesignificant birefringence, if the waveguide has a similar confinementfactor for both the TE and TM modes (the confinement factors beingdetermined by the refractive index and geometry of the waveguide) thephotodiode can be made substantially polarisation independent.

In the embodiments described above, the in-line photodiode is of thesame width as the waveguide leading to and from it. However, in somecases, it may be desirable to form the photodiode portion of a differentwidth (either wider or narrower) and provide transitional, taperedregions leading thereto.

The arrangements described above provide a number of advantages asindicated below:

-   -   (i) the light sensor region is automatically aligned with the        waveguide leading thereto as it is formed within the waveguide.    -   (ii) There is no possibility for the light sensor region to        subsequently move out of alignment with the waveguide (as can        happen with a hybridised light sensor).    -   (iii) The light does not have to pass through interfaces or        epoxy between the waveguide and the light sensor.    -   (iv) Fabrication is easier as the light sensor can be formed by        standard lithographic techniques and a separate component does        not have to be mounted and secured to the chip.    -   (v) The device is more rugged as it is of an integral        construction.

When the arrangement is used to monitor a signal being propagated alonga waveguide it also has the advantage that the proportion of lightabsorbed by the sensor can be accurately determined by the fabricationthereof and the fabrication techniques used are highly repeatable. Thisis in contrast to conventional arrangements used to tap off part of asignal, e.g. using directional couplers, Y-junctions etc. and measuringthe tapped off signal. Such devices are highly sensitive to fabricationtolerances which have a significant effect upon the proportion of lightthat is tapped off.

FIGS. 8-13 show other arrangements which can be used to provide in-lineoptical sensors and/or optical sensors that are integrated with theoptical circuit rather than being hybridised thereon.

FIG. 8 shows a cross-section through a waveguide 50 which usesmetallised areas to provide Schottky contacts which provideelectron-hole pairs from internal photoemission. Light transmitted alongthe waveguide is incident upon the metallised areas (as these areprovided on the walls of the waveguide) and provides charge carrierswithin the metal layer with sufficient energy to pass over the Schottkybarrier formed between the metal layer and the semiconductor material ofthe waveguide so releasing charge carriers within the waveguide. As inthe devices described above, these are then detected by a p-i-n diode.In the arrangement shown in FIG. 8, a vertical p-i-n diode is providedbetween p-doped regions 51, 52 on one or both sides of a rib waveguide50 and the metallised layer 53 provided on the upper surface of the rib.The metallised layer 53 thus serves both to form a Schottky barrier withthe semiconductor waveguide and as an electrical terminal of the p-i-ndiode for detecting the charge carriers generated within the device.

The polarisation dependence of such a device can be controlled byadjusting the dimensions of the metallised area 53. In anotherarrangement, the metallised area may partially cover both the upper andside surface(s) of the rib waveguide 50 as shown in the embodimentillustrated in FIG. 9.

Another alternative is to interdigitate the metal contacts as shown inthe plan view in FIG. 10. A first set of metallised areas 63A areprovided on the upper surface of the rib 60 and a second set ofmetallised areas 63B are provided on both the upper and side surfaces ofthe rib 60 (or just on the side surface(s) thereof). The length of themetallised areas in each set can be selected to balance the TE and TMabsorption by the metal contacts.

FIG. 11 is a perspective view of an in-line photodiode comprising a ribwaveguide 70 made of a first material, e.g. silicon, having a portion70A integrally formed therewith made of a second material, e.g. asilicon germanium alloy. The first material has a band gap which is toolarge for electrons excited by photons of the incoming light signal tomove into the conduction band. However, the second material is selectedto have a smaller band gap so that photons of the wavelength of light tobe detected are able to move into the conductive band. The secondmaterial is chosen so as to enable said portion to be integrally formedwith the remainder of the waveguide. The second material may, forinstance, be formed by diffusing in another material, e.g. Ge, and thenheat treating the portion to form an alloy between the first and secondmaterials, the composition of said alloy being selected to have a lowerband gap than that of the first material alone. Alternatively, a portionof the silicon waveguide can be etched away and a portion comprising asilicon-germanium alloy grown or deposited in its place. It will beapparent that the alloy should be such as to be integrally formed withthe adjacent portion of the waveguide albeit with some mis-match at theboundary therebetween due to the different materials. A p-i-n diode isprovided to detect the charge carriers generated in the Si/Ge portion ofthe waveguide. In the example shown, a longitudinal p-i-n diode isformed along the length of the waveguide, i.e. with a p- doped region 71formed in a part of the waveguide leading to the Si/Ge portion and ann-doped region 72 formed in a part of the waveguide downstream of theSi/Ge portion. Such an in-line p-i-n diode will have low efficiency dueto absorption of the optical signal by the p- and n-doped regions 71 and72 but this may be tolerated in some applications. Indeed, in somecases, the downstream doped region 72 may be extended so as to ensurethat it absorbs substantially all of the light signal received thereby,i.e. so as to form a beam dump downstream of the light sensor.

In other arrangements, particularly when it is required for at least asubstantial part of the signal to pass through the device, a lateral orvertical p-i-n diode arrangement such as those described above may beused.

FIGS. 12A and 12B show an arrangement having a lateral p-i-n diodeformed across a waveguide 70, p- and n- doped regions 71A and 72A beingprovided in the slab regions 74 on either side of the rib waveguide 70 aportion 70A of which, between the p- and n-doped regions 71A and 72A, isformed of a Si/Ge alloy.

FIG. 13 is a perspective view of another embodiment having a Schottkybarrier for generating charge carriers within a waveguide 80. In thiscase, a metallised region 81 is provided on the end face of a ribwaveguide (and surfaces of the waveguide adjacent the end face). Lighttransmitted along the waveguide 80 and incident on this metallised layer81 gives rise to the photoemisson of charge carriers into the end of thewaveguide. P- and n-doped regions 82 and 83 form a lateral p-i-n diodeacross the waveguide to detect these charge carriers. The metal layer 81may, for instance, be formed of platinum. A Schottky barrier is thusformed between the platinum and a layer of platinum silicide that arisesat the end of the silicon waveguide 80.

FIGS. 14-22 illustrate a further extension of the arrangements describedabove.

FIG. 14 shows a plan view of the type of photodiode described above,e.g. in relation to FIGS. 1 and 2 and FIG. 15 shows a cross-sectionthereof (which is similar to that shown in FIG. 2). As described above,the light sensor is formed as part of an integrated optical waveguide101, e.g. a rib waveguide formed in silicon, preferably in asilicon-on-insulator (SOI) substrate. The photodiode is arranged to beable to detect selected wavelengths, in particular wavelengths in therange 1.5-1.6 microns as widely used in telecommunications applications.Normally, silicon is transparent at these wavelengths but a portion 101Aof the waveguide 101 is modified so as to at least partially absorbwavelengths in this range leading to the generation of free chargecarriers in the waveguide. There are a variety of ways of doing this asdescribed above.

A p-i-n diode is provided to detect the presence of the charge carriersgenerated in this way. In the arrangement shown, p- and n-doped regionsare formed on opposite sides of the waveguide 101, which is nominallyintrinsic, and the p-i-n diode thus formed is used to generate anelectric signal which is indicative of the power of the light signalbeing sensed.

FIG. 15 shows the SOI substrate comprising a silicon layer 104 separatedfrom a supporting substrate 105 (typically also of silicon) by anoptical confinement layer 106, typically of silicon dioxide (whichperforms an insulating function in electrical applications—hence theterminology silicon-on-insulator). The rib waveguide 101 comprises a rib101B upstanding from a slab region 101C both of which are formed in thesilicon layer 104. The p- and n-doped regions 102, 103 are formed at thebase of recesses 101D, 101E formed in the silicon layer 104 andelectrical contacts 107, 108 provided thereon. A passivating oxide layer109 is provided over the silicon layer 104, except where the electricalcontact is made with the doped regions 102, 103.

As indicated above, other arrangements of p-i-n diode may be used orother forms of detecting means for detecting the charge carriersgenerated by the absorption of light of the selected wavelength.

In the embodiment to be described, such an arrangement is modified bythe provision of wavelength selective reflector means to reflect theselected wavelength or selected range of wavelengths repeatedly throughthe portion 101A so as to increase the absorption of the selectedwavelength or selected range of wavelengths and hence the level of thesignal generated, a proportion of the light being absorbed on each passthrough the portion 101A.

A preferred way of achieving this is to provide a Bragg grating in thewaveguide on each side of the photodiode. This is illustrated in FIG. 16which shows an input Bragg grating 111 at the input end of the waveguide101 and an output Bragg grating 112 at the output end of the waveguide101.

The gratings 111, 112 may be formed in the waveguide 101 using anelectron beam lithography technique in a known manner. The periodicityof the gratings 111, 112 is designed such that they selectively reflecta single wavelength or a band of wavelengths. For example, in awaveguide carrying the wavelengths λ₁, λ₂, . . . , λ_(N), the gratingmay have a periodicity such that it only reflects λ₂.

FIG. 17 shows a perspective view of a Bragg grating formed by etchinggrooves across the rib 101B of a rib waveguide.

The role of the pair of gratings 111, 112 is to tend to confine lightcorresponding to the grating wavelength λ_(n), or wavelength range,within the region 101A of the waveguide 101 where light of thatwavelength is absorbed, e.g. by the presence of gold atoms as describedabove. The two gratings 111, 112 together preferably form a Fabry-Perotcavity which selectively confines the wavelength λ_(n) between them thusgiving the detector a “resonant” condition for absorption at thatwavelength. Note that because a Fabry-Perot cavity is formed then thereis not a significant back-reflection of the incident light of theselected wavelength λ_(n) from the input Bragg grating, i.e. input lightof the selected wavelength is completely coupled into the cavity region.Light at the selected wavelength is thus forced to undergomultiple-passes of the waveguide photodetector and so, for thatparticular wavelength, the effective length of the photodetector isincreased. The function of such a Fabry-Perot cavity is well known, e.g.as described in Chapter 9, 2^(nd) Edition of the textbook “Optics” byHecht published by Addison Wesley Publishing Co.

It has been found experimentally that Bragg gratings with a line lengthof 3.4 μm and coupling coefficient, K, of 2.5 cm⁻¹ provide a goodpractical combination for process tolerant, high reflectivity Bragggratings. Using these design parameters, the grating reflectivities areexpected to be ˜0.99 and have been shown to attenuate the selectedwavelength by ˜20 dB in the transmission path. This implies that in alossless waveguide the selected wavelength would effectively undergo˜500 reflections based on the theory set out in “Optics” referred toabove. The implication of this is that the effective path length for theselected wavelength in the photodetector of FIG. 16 is multiplied by afactor of ˜500. This means that the wavelength-selective detector couldbe quite short in length yet have sufficient interaction with theselected wavelength, λ_(n), to allow significant photoabsorption. Ashort length is desirable for the waveguide photodetector, not only sothat the device is compact, but also because this minimises the unwantedabsorption of the wavelengths other than the selected one, λ_(n).

The arrangement shown in FIG. 16 can be extended to two or more suchlight sensors either in series in the same waveguide or in parallel inseparate waveguides. Whilst some compromise in performance will have tobe accepted when used in series (to allow more than one wavelength topass through the upstream sensor or sensors whilst maintainingsufficient wavelength selectivity within each sensor), such arrangementscould be useful in certain devices. One possibility is for anarrangement performing the role of an optical channel monitor (OCM). Atypical, known form of OCM is shown in FIG. 18 and comprises an inputwaveguide 120, a first free propagation region 121 leading to an arrayedwaveguide grating (AWG) 122, the output of which crosses a second freepropagation region 123 to a plurality of output waveguides 124 whichlead to respective photodetectors 125.

Light enters the OCM device via the input waveguide 120. Typically, theinput light will be made up of a spectrum of different wavelengths: λ₁,. . . , λ_(N). The input waveguide delivers the light into the AWG 122which disperses the light into its component wavelengths. The lightemerges from the AWG 122 onto a circle R. The point on R at which thelight is focused is dependent upon its wavelength. Output waveguides 124are provided along the circle R and these waveguides 124 transport thelight to the edge of the optical chip. Each output waveguide 124 carriesa different wavelength of light. The example shown has 5 waveguides butother arrangements may have fewer waveguides or more waveguides.

A photodiode array 125 is located at the edge of the chip where theoutput waveguides 124 terminate. Typically, this photodiode array 125 ismade from a different material to the optical chip; for example, theoptical chip may be a SIMOX silicon wafer while the photodiode array maybe a III-V semiconductor. The photodiode array 125 must be carefullyoptically aligned to the output waveguide facets and then bonded inplace by a suitable epoxy; a process known as hybridisation. Each pixelof the photodiode array 125 is used to detect the light from one of theoutput waveguides 124 thus providing a measure of the optical power at aparticular wavelength.

A plurality of light sensors can be used to provide a function similarto that of an OCM function. One way of doing this is to provide a sensoron each of the waveguides 124, each arranged to sense the wavelengthreceived by the respective waveguide. Another way is to provide aplurality of light sensors in series as shown in FIG. 19. In eithercase, each light sensor comprising a detection region with Bragggratings on the input and output sides thereof, each pair of Bragggratings being arranged reflect a different wavelength or band ofwavelengths.

FIG. 19 shows a series of light sensors in which light enters the seriesvia the waveguide on the left. The light, which is composed of thewavelengths λ₁, λ₂, . . ., λ_(N), passes into a firstwavelength-selective detector 130 ₁ which has its Bragg grating pair setso as to preferentially photodetect one wavelength or a range ofwavelengths. If the overall reflectivity of the grating is relativelylow, the other wavelengths present in the optical signal will be largelyunaffected by their passage through this first detector and willcontinue to propagate along the waveguide 101 to further waveguidephotodetectors 130 ₂, . . . , 130 _(n). These further detectors aredesigned so as to selectively measure each of the remaining wavelengthsor ranges of wavelengths that make up the signal so that, eventually,after a passage through N detectors (where N is the number ofwavelengths in the network) every wavelength has had its power levelsampled.

In order that light reflected by the detectors does not return tosource, the input may be fitted with a circulator to allow the signalsto continue to their destination or to be dumped or passed on forfurther measurement, e.g. to another circulator and another wavelengthspecific sensor of the type described (but tuned to another wavelength).A cascade of such circulators and wavelength specific sensors may beprovided to enable a plurality of wavelengths to be monitored.

The system could be designed to optimise reflection (i.e. rejection) ofwavelengths or reflectivity of the grating could be lower. In this case,ideally all of the optical power carried by the wavelength λ₁ orwavelength range would be absorbed at the first photodetector, with theother wavelengths, λ₁, λ₂, . . . , λ_(N), being reflected. However, thearrangement could be designed so as to photoabsorb only a smallfraction, say 1-5%, of the total optical power in the waveguide. This isa common requirement for optical monitoring of an optical network. Infact, the conventional OCM of FIG. 18 would typically be employed in anetwork to monitor the spectral composition of the light output from amajor network element such as an optical add-drop multiplexer (OADM) oran erbium-doped amplifier (EDFA) 140 (see FIG. 20). In such anapplication, the conventional OCM 141 would be preceded by a tap coupler142 before its input that would extract typically 1-10% of the totaloptical power from the main network connection for monitoring purposes,while allowing the remainder to propagate onwards in the network (asillustrated in FIG. 20).

Where the reflectivity of the Bragg gratings is high (typically >0.99)then, effectively, the first Fabry-Perot cavity could monitor thecombined power of a range of wavelengths, all other wavelengths beingreflected. The light of the wavelength range monitored by the firstsensor would be transmitted along the waveguide to the next Fabry-Perotcavity detector where a narrower range of wavelengths (or a specificwavelength) could be monitored.

An advantage of the arrangement of FIG. 19 is that if the photodetectorsare each designed only to absorb a small fraction of the total opticalpower (˜1-10%) then the tap coupler 142 of FIG. 20 is no longerrequired. Implementing such a small tap using evanescent couplers isdifficult as the tap fraction and polarisation dependent losses thereofare difficult to control.

Further advantages of the arrangement of FIG. 19 over the moreconventional approach include the compact and monolithic nature of thedevice. In FIG. 19 the power tap, wavelength separation andphotodetection functions are combined into a single monolithic chiprather than being realised as separate elements. In contrast, in theconventional approach, the tap coupler may often be implemented using afibre coupler, the wavelength separation may be a silicon AWG, e.g.formed on a silicon-on-insulator (SOI) chip, and the photodiode arraymay be a III-V semiconductor array that is hybridised onto the SOI chipusing epoxy adhesive.

The AWG-based OCM shown in FIG. 18 also typically occupies a largeamount of space on the optical chip; a chip that is roughly 5×7 cm wouldnot be unusual. This is because of a number of contributing factors,including the length required for the free propagation regions 121, 123of the AWG, the length of the AWG 122 itself and the space for thefan-out of the output waveguides 124 to the wide hybridised photodiodearray 125.

In contrast, the arrangement of FIG. 19 is based upon a single waveguideand so can be made very compact. The size limitations depend mainly uponthe total length of the device and the minimum bend radius of thewaveguide. The device length will depend upon, among other factors, thesensitivity of the individual photodetectors and the number ofwavelengths (N) in the system while the minimum bend radius will dependupon the etch-depth of the photonics.

The arrangement shown in FIG. 19 could be modified to take up a compact,snake-like layout such as that shown in FIG. 21. In this layout, thedetection elements 150 ₁, 150 ₂ . . . 150 _(n) may be separated fromeach other by isolation features 151 of the type disclosed inPCT/GB01/04191, in particular n-i-p-i-n dopant isolation and etchedtrenches. These measures reduce both optical and electrical crosstalkbetween the photodetectors.

Other similar compact layouts can also be envisaged, e.g. running thewaveguide on a spiral path or minimising the size further by positioningthe monitor sections on the waveguide bends. If the detection element(gratings plus diode) is 10 mm long and 250 μm wide then, allowing forbends, a 40-channel OCM would occupy ˜25×10 mm, i.e. ˜ 1/14 of the areaof a conventional OCM. This opens up the possibility of integrating theOCM functionality onto the same optical chip as other photonicfunctions. For example, the configuration of FIG. 21 could be fabricatedon a single optical chip. This would greatly simplify fabrication,create a more compact device and reduce chip-to-fibre interface losses.

As mentioned earlier, when the full spectrum of light passes into thefirst detector element, which is set to sample the wavelength λ₁, theother wavelengths λ₂, . . . , λ_(N) may also undergo a single-pass ofthis first detector. As a result, they also contribute to the totalphotocurrent of this element. This is undesirable as it represents acrosstalk component of the photocurrent. The resonant detection of λ₁will mean that, in most circumstances, the effect of the otherwavelengths is relatively small but this is not always the case. Aparticularly bad case would be a DWDM system comprising a large numberof channels, say many tens of channels, in which the λ₁ wavelength had alow optical power while all the other channels had high powers. Thisproblem could be dealt with by signal processing electronics. Forexample, if λ₁ was a low power signal but λ₂ was a high power signal(thus giving significant crosstalk at the λ₁ detector), the large λ₂power would be picked up at the λ₂ detector. The reading on the λ₂detector could then be used to deduce the fraction of the photocurrentfrom the λ₁ detector that was a result of crosstalk from λ₂. Thiscrosstalk could then be subtracted from the signal in digital signalprocessing (DSP) electronics to give a true reading of the power in λ₁.Likewise, the crosstalk from the other spectral components could beremoved from the data.

Another potential problem, which does not arise in a conventional OCM,is that of saturation of the detectors. Consider again the λ₁ detector'sbehaviour. If the number of wavelengths in the system, N, is large andmost DWDM channels are operating at their maximum optical power, thenthe total optical power through λ₁ will be very high. This opens up thepossibility that the total power may be large enough to push the λ₁detector towards the saturation region of its operation, particularly ifthe λ₁ signal is also large. The detector would therefore startoperating in a non-linear region of its response curve and would thusgive inaccurate power measurements. This would mean that the range ofpowers over which the OCM could be used would be reduced, i.e. thedynamic range of the device would be decreased. To avoid thisdifficulty, the photodetectors can be designed with photosensitivitiesappropriate to the anticipated powers that would be measured and tooptimise the Fabry-Perot cavity design so as to make thewavelength-selectivity of the detectors as high as possible. Thephotosensivity of the detectors could be adjusted by altering the levelsof the dopant, e.g. gold, used to absorb the light. For example, in anetwork where it was known that the signal λ₂ was always going tooperate over a higher range of powers than λ₁ then the level of dopingin the λ₂ detector could be reduced in comparison to the A detector,thus making it less responsive to the light incident upon it.

Another possibility for dealing with detectors with different dynamicranges would be to introduce variable optical attenuators (VOAs) intothe signal path. The VOAs would typically be absorption VOAs which wouldessentially consist of PIN diodes placed laterally across the waveguidewith a length appropriate to the required attenuation and available chipspace. A VOA will attenuate the light that is input into all of thewavelength-selective detectors that follow it and so the most likelylocation for a VOA is at the front on the detector series, before the λ₁detector, where it would attenuate all the input light uniformly. Asecond possibility would be to have VOAs positioned between eachwavelength dependent detector which would give some flexibility, suchthat the ordering of the wavelength detectors along the waveguide lengthwould be such that the most sensitive detectors would be placed farthestfrom the input. Thus, the light level would be progressively attenuatedalong the waveguide path as it passes into progressively more sensitivedetectors. In this multiple-VOA configuration, the wavelengthsensitivity of the VOAs could be exploited to enhance the wavelengthselectivity of the detector. For example, PIN diode absorption VOAs thatuse the free carrier dispersion effect as the basis of their operationare more effective at attenuating longer wavelength signals. This meansthat there would be enhanced selection of shorter wavelength signals atthe detectors placed at the downstream end of the series, so it would bepreferable to place longer wavelength detectors closer to the input atthe upstream end of the series. Of course, where the signal is expectedto be propagated onwards in the network after the photodetector series(i.e. the application is a tap-monitor application rather than 100%detection) then it may be inappropriate to use VOAs.

A further extension of the above, would be to make thewavelength-selective detector or detectors dynamically tuneable. Thismay be done, for example, by using heater elements 160 placed adjacentto the Bragg gratings 161 and/or to the detector elements 162 as shownin FIG. 22. The heaters 160 can be used to change the temperature of thesilicon rib waveguide which means that its refractive index will also bealtered via the thermo-optic effect. This can be used to change thewavelength selected by the detector.

A dynamically tuneable wavelength-selective detector would enableincreased functionality over the suggestions already made above. Itcould, for example, be used as a single tap monitor that could be set toa wavelength chosen by the network management system. Anotherpossibility is that it could be used to dynamically scan across acontinuous spectral range for power monitoring purposes. This wouldallow the function of an OCM (described above) to be implemented in acompact device with the advantage that the entire wavelength spectrumcould be sampled. As stated previously, one of the problems of the knownOCM shown in FIG. 18 is that the details of the spectral content arelost because the optical power measured by the pixels of the photodiode125 only record the power in the respective output waveguides 124. Theoutput waveguides 124 effectively sample the power in the particularregion of the spectrum that corresponds to the channel bandwidth, ratherthan the detailed spectral components that make up the total power foundwithin that spectral regional. A scanned wavelength-selective detectorwould instead measure a continuum of wavelengths. This is particularlydesirable for monitoring the power in high bit rate systems, say 40 Gbpsor greater, where OCMs suffer from intrinsic crosstalk due to the highbit rate leading to the spectral content of the signal spreading intoadjacent channels.

Whilst Bragg gratings are preferred, other forms of wavelength selectivereflectors may be used.

As described above, the reflector preferably reflects a single selectedwavelength but in other cases it may be desirable to reflect a narrowband of selected wavelengths, to reflect a plurality of selectedwavelengths or to reflect a broader band of selected wavelengths whilstrejecting wavelengths outside this band.

The p-i-n diode used to detect the charge carriers generated in thesensor may be arranged in other ways, e.g. laterally, vertically,longitudinally etc, so long as the p- and n-doped regions are positionedso that an electrical signal is generated across the diode in responseto the generation of free charge carriers in the sensor. Other forms ofdetector means may also be used to detect the presence of the freecharge carriers.

As described above, a portion of the light path preferably absorbs lightof a selected wavelength to generate free charge carriers. This can beachieved in a variety of ways including the provision of one or more ofthe following in said portion: amorphous material, polycrystallinematerial, an alloy, a material which has been doped and/or had defectsformed therein to provide deep band gap states within the band gapthereof and a material having isolated regions, e.g. quantum dots,therein which generate charge carriers when illuminated by the selectedwavelength. Alternatively, said portion of the path is provided with oneor more metallised areas which form a Schottkey barrier with thematerial of said portion.

The p-i-n photodiode structure described herein may also be used inrelation to other types of waveguide or in any arrangement in whichlight transmitted along a waveguide is to be monitored or sensed, e.g.at the end of a waveguide or within a resonant cavity.

1. An integrated optical waveguide having an in-line light sensorintegrally formed therewith, comprising: a first part of the waveguideleading to a photodiode portion thereof; a second part of the waveguideleading away from the photodiode portion, a material from which saidfirst and second parts of the waveguide are formed having an energy bandgap a magnitude of which corresponds to absorption of photons of a firstwavelength, the photodiode portion comprising one or more regions oflight absorbing material within the waveguide arranged to absorb a minorproportion of light of one or more selected wavelengths transmittedalong the waveguide such that a major proportion of the light passesthrough to the second part of the waveguide to thereby generate freecharge carriers within the photodiode portion of the waveguide, thephotodiode portion having a modification that introduces deep band gaplevels therein so as to provide at least partial absorption of photonsof a selected wavelength or wavelength band greater than said firstwavelength; detecting means for detecting the presence of said freecharge carriers; and in which the material of the waveguide and/or thedimensions thereof are selected so as to provide similar confinementfactors for both the TE and TM modes whereby the detection of lightthereby is substantially polarization independent.
 2. A waveguide asclaimed in claim 1 in which the modification includes elementalimpurities within a crystalline structure in the photodiode portion. 3.A waveguide as claimed in claim 1 in which the detecting means comprisesa p-i-n diode comprising a p-doped region, and an n-doped region inelectrical contact with a nominally intrinsic region begin located sothe majority of light transmitted along the waveguide passestherethrough.
 4. A waveguide as claimed in claim 3 in which thenominally intrinsic region is relatively lightly doped with p-dopantadjacent said p-doped region and n-dopant adjacent said n-doped region.5. A waveguide as claimed in claim 3, wherein the waveguide is a ribwaveguide comprising a rib projecting from a slab region, and whereinthe p- and n-doped regions are formed on opposite sides of the ribwaveguide.
 6. A waveguide as claimed in claim 5 in which the p-dopedand/or n-doped regions are formed at the base of one or more recessesformed in the slab region.
 7. A waveguide as claimed in claim 3, whereinthe waveguide is a rib waveguide comprising a rib projecting from a slabregion, and wherein the p-doped region is formed on one side or bothsides of the rib waveguide and the n-doped region is formed on top ofthe rib waveguide, or vice versa.
 8. A waveguide as claimed in claim 1which is a rib waveguide comprising a rib projecting from a slab region.9. A. waveguide as claimed in claim 8 in which said photodiode portionis, at least partially, within the rib of the rib waveguide.
 10. Awaveguide as claimed in claim 1 formed on a silicon-on-insulator chip.11. A waveguide as claimed in claim 1 in which the selected wavelengthband is around 1.3 or 1.5 microns.
 12. A waveguide as claimed in claim1, where the in-line light sensor is one of a plurality of in-line lightsensors included in the waveguide and each in-line light sensor isarranged to be sensitive to a different wavelength or wavelength band.13. An integrated optical waveguide having an in-line light sensorintegrally formed therewith, comprising: a first part of the waveguideleading to a photodiode portion thereof; a second part of the waveguideleading away from the photodiode portion, a material from which saidfirst and second parts of the waveguide are formed having an energy bandgap a magnitude of which corresponds to absorption of photons of a firstwavelength, the photodiode portion comprising one or more regions oflight absorbing material within the waveguide arranged to absorb a minorproportion of light of one or more selected wavelengths transmittedalong the waveguide such that a major proportion of the light passesthrough to the second part of the waveguide to thereby generate freecharge carriers within the photodiode portion of the waveguide, thephotodiode portion having a modification that introduces deep band gaplevels therein so as to provide at least partial absorption of photonsof a selected wavelength or wavelength band greater than said firstwavelength; detecting means for detecting the presence of said freecharge carriers; and a wavelength selective reflector means beingarranged to reflect light of said selected wavelength or range ofwavelengths so it passes repeatedly through the photodiode.
 14. Awaveguide as claimed in claim 13 in which the modification includeselemental impurities in a crystalline structure in the photodiodeportion.
 15. A waveguide as claimed in claim 13 in which the detectingmeans comprises a p-i-n diode comprising a p-doped region, and ann-doped region in electrical contact with a nominally intrinsic regionbegin located so the majority of light transmitted along the waveguidepasses therethrough.
 16. A waveguide as claimed in claim 15 in which thenominally intrinsic region is relatively lightly doped with p-dopantadjacent said p-doped region and n-dopant adjacent said n-doped region.17. A waveguide as claimed in claim 15, wherein the waveguide is a ribwaveguide comprising a rib projecting from a slab region, and whereinthe p- and n-doped regions are formed on opposite sides of the ribwaveguide.
 18. A waveguide as claimed in claim 17 in which the p-dopedand/or n-doped regions are formed at the base of one or more recessesformed in the slab region.
 19. A waveguide as claimed in claim 15,wherein the waveguide is a rib waveguide comprising a rib projectingfrom a slab region, and wherein the p-doped region is formed on one sideor both sides of the rib waveguide and the n-doped region is formed ontop of the rib waveguide, or vice versa.
 20. A waveguide as claimed inclaim 13 which is a rib waveguide comprising a rib projecting from aslab region.
 21. A waveguide as claimed in claim 20 in which saidphotodiode portion is, at least partially, within the rib of the ribwaveguide.
 22. A waveguide as claimed in claim 13 formed on asilicon-on-insulator chip.
 23. A waveguide as claimed in claim 13 inwhich the selected wavelength band is around 1.3 or 1.5 microns.
 24. Awaveguide as claimed in claim 13 in which the reflective means comprisesfirst and second reflectors.
 25. A waveguide as claimed in claim 24 inwhich the first and second reflectors are provided in the first andsecond parts of the waveguide on opposite sides of the diode portion.26. A waveguide as claimed in claim 24 in which at least one of thefirst and second reflectors comprises a Bragg grating.
 27. A waveguideas claimed in claim 13 in which the in-line light sensor is tunable soas to be sensitive to one or more selected wavelengths or wavelengthbands.
 28. A waveguide as claimed in claim 27 in which first wavelengthcontrol means are provided to adjust the wavelength or band ofwavelengths reflected by the reflector means.
 29. A waveguide as claimedin claim 27 in which second wavelength control means are provided toadjust the wavelength or band of wavelengths absorbed within the diodeportion.
 30. A waveguide as claimed in claim 27 in which the in-linelight sensor can be scanned over a range of wavelengths to provide aspectral analysis of the light received.
 31. A waveguide as claimed inclaim 13 where the in-line light sensor is one of a plurality of in-linelight sensors included in the waveguide and each in-line light sensor isarranged to be sensitive to a different wavelength or wavelength band.32. An integrated optical waveguide having an in-line light sensorintegrally formed therewith, comprising: a first part of the waveguideleading to a photodiode portion thereof; a second part of the waveguideleading away from the photodiode portion, a material from which saidfirst and second parts of the waveguide are formed having an energy bandgap a magnitude of which corresponds to absorption of photons of a firstwavelength, the photodiode portion comprising one or more regions oflight absorbing material within the waveguide arranged to absorb a minorproportion of light of one or more selected wavelengths transmittedalong the waveguide such that a major proportion of the light passesthrough to the second part of the waveguide to thereby generate freecharge carriers within the photodiode portion of the waveguide, thephotodiode portion having a modification that introduces deep band gaplevels therein so as to provide at least partial absorption of photonsof a selected wavelength or wavelength band greater than said firstwavelength; detecting means for detecting the presence of said freecharge carriers; and an optical attenuator for attenuating the lightpassing through the in-line light sensor, wherein said attenuator is avariable optical attenuator.
 33. A waveguide as claimed in claim 32 inwhich the modification includes elemental impurities in a crystallinestructure in the photodiode portion.
 34. A waveguide as claimed in claim32 in which the detecting means comprises a p-i-n diode comprising ap-doped region, and an n-doped region in electrical contact with anominally intrinsic region begin located so the majority of lighttransmitted along the waveguide passes therethrough.
 35. A waveguide asclaimed in claim 34 in which the nominally intrinsic region isrelatively lightly doped with p-dopant adjacent said p-doped region andn-dopant adjacent said n-doped region.
 36. A waveguide as claimed inclaim 34, wherein the waveguide is a rib waveguide comprising a ribprojecting from a slab region, and wherein the p- and n-doped regionsare formed on opposite sides of the rib waveguide.
 37. A waveguide asclaimed in claim 34, wherein the waveguide is a rib waveguide comprisinga rib projecting from a slab region, and wherein the p-doped region isformed on one side or both sides of the rib waveguide and the n-dopedregion is formed on top of the rib waveguide, or vice versa.
 38. Awaveguide as claimed in claim 36 in which the p-doped and/or n-dopedregions are formed at the base of one or more recesses formed in theslab region.
 39. A waveguide as claimed in claim 32 which is a ribwaveguide comprising a rib projecting from a slab region.
 40. Awaveguide as claimed in claim 39 in which said photodiode portion is, atleast partially, within the rib of the rib waveguide.
 41. A waveguide asclaimed in claim 32 formed on a silicon-on-insulator chip.
 42. Awaveguide as claimed in claim 32 in which the selected wavelength bandis around 1.3 or 1.5 microns.
 43. A waveguide as claimed in claim 32where the in-line light sensor is one of a plurality of in-line lightsensors included in the waveguide and each in-line light sensor isarranged to be sensitive to a different wavelength or wavelength band.44. A waveguide as claimed in claim 43 in which the in-line lightsensors are arranged in series along a serpentine light conductive path.45. A waveguide as claimed in claim 44 formed on a substrate, saidsubstrate having optical and/or electrical isolation devices formedtherein positioned so as to assist in optically and/or electricallyisolating different portions of said serpentine path from each other.46. An integrated optical waveguide having an in-line light sensorintegrally formed therewith, comprising: a first part of the waveguideleading to a photodiode portion thereof; a second part of the waveguideleading away from the photodiode portion, a material from which saidfirst and second parts of the waveguide are formed having an energy bandgap a magnitude of which corresponds to absorption of photons of a firstwavelength, the photodiode portion comprising one or more regions oflight absorbing material within the waveguide arranged to absorb a minorproportion of light of one or more selected wavelengths transmittedalong the waveguide such that a major proportion of the light passesthrough to the second part of the waveguide to thereby generate freecharge carriers within the photodiode portion of the waveguide, thephotodiode portion having a modification that introduces deep band gaplevels therein so as to provide at least partial absorption of photonsof a selected wavelength or wavelength band greater than said firstwavelength; detecting means for detecting the presence of said freecharge carriers; and two or more in-line light sensors arranged inseries along a serpentine light conductive path.
 47. A waveguide asclaimed in claim 46 in which the modification includes elementalimpurities in a crystalline structure in the photodiode portion.
 48. Awaveguide as claimed in claim 46 in which the detecting means comprisesa p-i-n diode comprising a p-doped region, and an n-doped region inelectrical contact with a nominally intrinsic region begin located sothe majority of light transmitted along the waveguide passestherethrough.
 49. A waveguide as claimed in claim 48 in which thenominally intrinsic region is relatively lightly doped with p-dopantadjacent said p-doped region and n-dopant adjacent said n-doped region.50. A waveguide as claimed in claim 46 which is a rib waveguidecomprising a rib projecting from a slab region.
 51. A waveguide asclaimed in claim 46, wherein the waveguide is a rib waveguide comprisinga rib projecting from a slab region, and wherein the p- and n-dopedregions are formed on opposite sides of the rib waveguide.
 52. Awaveguide as claimed in claim 51 in which the p-doped and/or n-dopedregions are formed at the base of one or more recesses formed in theslab region.
 53. A waveguide as claimed in claim 52 in which saidphotodiode portion is, at least partially, within the rib of the ribwaveguide.
 54. A waveguide as claimed in claim 46, wherein the waveguideis a rib waveguide comprising a rib projecting from a slab region, andwherein the p-doped region is formed on one side or both sides of therib waveguide and the n-doped region is formed on top of the ribwaveguide, or vice versa.
 55. A waveguide as claimed in claim 46 formedon a silicon-on-insulator chip.
 56. A waveguide as claimed in claim 46in which the selected wavelength band is around 1.3 or 1.5 microns. 57.A waveguide as claimed in claim 46 in which each in-line light sensor isarranged to be sensitive to a different wavelength or wavelength band.58. A waveguide as claimed in claim 46 formed on a substrate, saidsubstrate having optical and/or electrical isolation devices formedtherein positioned so as to assist in optically and/or electricallyisolating different portions of said serpentine path from each other.59. A waveguide as claimed in claim 46 in which each of the in-linelight sensors has a variable optical attenuator in series therewith. 60.An integrated optical waveguide having an in-line light sensorintegrally formed therewith, comprising: a first part of the waveguideleading to a photodiode portion thereof; a second part of the waveguideleading away from the photodiode portion, a material from which saidfirst and second parts of the waveguide are formed having an energy bandgap a magnitude of which corresponds to absorption of photons of a firstwavelength, the photodiode portion comprising one or more regions oflight absorbing material within the waveguide arranged to absorb a minorproportion of light of one or more selected wavelengths transmittedalong the waveguide such that a major proportion of the light passesthrough to the second part of the waveguide to thereby generate freecharge carriers within the photodiode portion of the waveguide, thephotodiode portion having a modification that introduces deep band gaplevels therein so as to provide at least partial absorption of photonsof a selected wavelength or wavelength band greater than said firstwavelength; detecting means for detecting the presence of said freecharge carriers; and the in-line light sensor is one of a plurality ofin-line light sensors included in the waveguide, the in-line lightsensors arranged in series or in parallel, and each light sensor havinga variable optical attenuator in series therewith.
 61. A waveguide asclaimed in claim 60 in which the modification includes elementalimpurities in a crystalline structure in the photodiode portion.
 62. Awaveguide as claimed in claim 61, wherein the waveguide is a ribwaveguide comprising a rib projecting from a slab region, and whereinthe p- and n-doped regions are formed on opposite sides of the ribwaveguide.
 63. A waveguide as claimed in claim 62 in which the p-dopedand/or n-doped regions are formed at the base of one or more recessesformed in the slab region.
 64. A waveguide as claimed in claim 63 inwhich said photodiode portion is, at least partially, within the rib ofthe rib waveguide.
 65. A waveguide as claimed in claim 61 wherein thewaveguide is a rib waveguide comprising a rib projecting from a slabregion, and wherein the p-doped region is formed on one side or bothsides of the rib waveguide and the n-doped region is formed on top ofthe rib waveguide, or vice versa.
 66. A waveguide as claimed in claim 60in which the detecting means comprises a p-i-n diode comprising ap-doped region, and an n-doped region in electrical contact with anominally intrinsic region begin located so the majority of lighttransmitted along the waveguide passes therethrough.
 67. A waveguide asclaimed in claim 66 in which the nominally intrinsic region isrelatively lightly doped with p-dopant adjacent said p-doped region andn-dopant adjacent said n-doped region.
 68. A waveguide as claimed inclaim 60 in which is a rib waveguide comprising a rib projecting from aslab region.
 69. A waveguide as claimed in claim 60 formed on asilicon-on-insulator chip.
 70. A waveguide as claimed in claim 60 inwhich the selected wavelength band is around 1.3 or 1.5 microns.